Postgres10高性能開發（6）鎖及其檢查

鎖類型

參考文檔：https://www.postgresql.org/docs/10/explicit-locking.html

表級鎖

鎖名稱	引發操作
ACCESS SHARE 訪問共享鎖	SELECT 任何讀取不修改表的查詢都將獲取本鎖
ROW SHARE 行共享鎖	SELECT FOR UPDATE SELECT FOR SHARE
ROW EXCLUSIVE 行排他鎖	UPDATE DELETE INSERT
SHARE UPDATE EXCLUSIVE 共享更新排他鎖	VACUUM（不含FULL）ANALYZE CREATE INDEX CONCURRENTLY CREATE STATISTICS 部分ALTER TABLE
SHARE 共享鎖	CREATE INDEX（無CONCURRENTLY）
SHARE ROW EXCLUSIVE 共享行級排它鎖	CREATE COLLATION CREATE TRIGGER 部分ALTER TABLE
EXCLUSIVE 排它鎖	REFRESH MATERIALIZED VIEW CONCURRENTLY
ACCESS EXCLUSIVE 訪問排他鎖	DROP TABLE TRUNCATE REINDEX CLUSTER VACUUM FULL REFRESH MATERIALIZED VIEW (不含 CONCURRENTLY)

Requested Lock Mode	Current Lock Mode
	ACCESS SHARE	ROW SHARE	ROW EXCLUSIVE	SHARE UPDATE EXCLUSIVE	SHARE	SHARE ROW EXCLUSIVE	EXCLUSIVE	ACCESS EXCLUSIVE
ACCESS SHARE								X
ROW SHARE							X	X
ROW EXCLUSIVE					X	X	X	X
SHARE UPDATE EXCLUSIVE				X	X	X	X	X
SHARE			X	X		X	X	X
SHARE ROW EXCLUSIVE			X	X	X	X	X	X
EXCLUSIVE		X	X	X	X	X	X	X
ACCESS EXCLUSIVE	X	X	X	X	X	X	X	X

行級鎖

鎖定查找到行，阻止對行進行某些操作

鎖	阻止操作
FOR UPDATE	UPDATE DELETE SELECT FOR UPDATE SELECT FOR NO KEY UPDATE SELECT FOR SHARE SELECT FOR KEY SHARE
FOR NO KEY UPDATE	UPDATE DELETE SELECT FOR UPDATE SELECT FOR NO KEY UPDATE SELECT FOR SHARE
FOR SHARE	UPDATE DELETE SELECT FOR UPDATE SELECT FOR NO KEY UPDATE
FOR KEY SHARE	UPDATE DELETE SELECT FOR UPDATE

Requested Lock Mode	Current Lock Mode
	FOR KEY SHARE	FOR SHARE	FOR NO KEY UPDATE	FOR UPDATE
FOR KEY SHARE				X
FOR SHARE			X	X
FOR NO KEY UPDATE		X	X	X
FOR UPDATE	X	X	X	X

檢查和處理鎖

參考文檔：https://www.postgresql.org/docs/10/view-pg-locks.html

死鎖

如果數據庫發現死鎖，數據庫將自動發現並終止死鎖

pg_locks

pg_locks視圖記錄當前進程的鎖定情況，主要字段包括

• locktype：鎖定類型，包括 relation（表）, extend, page, tuple（元組）, transactionid（事物ID）, virtualxid, object, userlock, or advisory
• database：鎖所在數據庫ID，關聯到 pg_database.oid
• relation：鎖所在的表ID，關聯到pg_class.oid
• pid：持有鎖或等待鎖的進程ID
• mode：持有或等待的鎖名稱
• granted：True表示數已持有，false表示在等待持有

查找當前的鎖

select l.mode,granted,l.locktype,l.pid,query,query_start,db.datname,relname,l.*
from pg_locks l LEFT JOIN pg_stat_activity a on a.pid = l.pid
LEFT JOIN pg_database db on oid = database
LEFT JOIN pg_class on oid = relation
where 1=1
--and l.pid=20139
--and database=(select oid from pg_database where datname='test')
--and relation=(select oid from pg_class where relname='report_user')

排查鎖定方法

首先查找一直在等待的鎖

select l.mode,granted,l.locktype,l.pid,query,query_start,db.datname,relname,l.*
from pg_locks l LEFT JOIN pg_stat_activity a on a.pid = l.pid
LEFT JOIN pg_database db on oid = database
LEFT JOIN pg_class on oid = relation
where granted = false

在根據鎖定類型，鎖定對象查找當前持有這個鎖的PID及SQL，例如

select l.mode,granted,l.locktype,l.pid,query,query_start,db.datname,relname,l.*
from pg_locks l LEFT JOIN pg_stat_activity a on a.pid = l.pid
LEFT JOIN pg_database db on oid = database
LEFT JOIN pg_class on oid = relation
where transactionid=81401737

判斷已獲取的鎖是否與當前鎖衝突，如果衝突，可以使用其PID停止該數據庫連接以釋放鎖
例如：PID爲31292

select pg_terminate_backend(31292)